The Study Of High Plasma Density Experiments On EAST

Plasma density is one of the most important parameters in the fusion plasma research. High density operation is beneficial to fulfil the ignition conditions and it should be the basic requirement of the fusion plant in the future. H-mode plasma with density up to ～0.85nGW is the baseline scenario of ITER. The demonstration of long pulse operation of tokamak plasmas at a high density level while maintaining a good energy confinement is crucial for ITER in the future. Increasing the density in a tokamak is limited by the so-called density limit, which is generally performed as an appearance of disruption causing loss of plasma confinement, or a degradation of high confinement mode which could further lead to a Hâ†'L transition. Despite large experimental and theoretical work, the underlying physics is not yet fully understood. EAST is a fully superconducting tokamak which has a similar configuration and operation scenario to ITER. Therefore, it is very meaningful to investigate high density operation and density limit on EAST.In a tokamak, energy and particles loss are undergoing during the plasma discharge. Therefore, fueling system is needed to deliver the working gas to the plasma. EAST has developed three kinds of fueling system:gas puffing, pellet injection and supersonic molecular beam injection (SMBI). The principle of SMBI is that a supersonic beam will be formed when subsonic gas injects into a high vacuum through a Laval nozzle. SMBI system combines the advantages of simple structure and rapid time response. As indicated by tests and fueling experiments, the velocity of SM beam is between 400-1200 m/s, the response time for density control is 2-6ms. The fueling efficiency of SMBI is about 15%-30%, which is 2 factors higher than gas puffing, and it decreases with the growth of density. SMBI is a very good plasma density feedback actuator, and it shows a high controllability at low or high density. Compared with gas puffing feedback, the gas input, particle retention and global recycling efficiency are lower with SMBI feedback. Moreover, SMBI successfully mitigated the ELMs in H-mode. With interval 8-12ms SMBI pulses, the ELMs’ frequency increased a factor a 5-10, and their amplitude become lower with longer SMBI pulse. In ELM-free H-mode, a string of ELMs was triggered by using 12ms SMBI pulse.In EAST, density limit discharge could be achieved generally by strong or just moderate fueling rate. In this thesis, L-mode and H-mode density limit discharges on EAST were analyzed. The generally pattern of L-mode density limit could be described as follows:when the temperature at the divertor targets drops to the vicinity of several eV, the plasma starts to detach from the targets, initially close to the separatrix; at the nearby time, a highly radiating region, denoted as ’MARFE’ (multifaceted asymmetric radiation from edge), moves out of the divertor towards the X-point and, if the bulk density continues to rise, the MARFE enters the bulk plasma, precipitating the growth of MHD (magnetohydrodynamic) activity and ultimately disruption. MARFE does not necessarily appear prior to a density limit disruption. However, the enhancement of edge cooling is still working as the main factor which lead to the disruption. Through the application of advanced wall conditioning methods and improvement of auxiliary heating power, the density operation space has been significantly extended on EAST over the past five years.In the NBI H-mode density limit discharge, experimental data indicated that high Z impurities (mainly Fe and Cu) are accumulated in the core plasma in the later stage of H-mode. The excessive central radiation led to the decrease of core temperature and further caused the Hâ†'L transition. The constancy of the edge density gradient probably indicates a critical limit at the pedestal region, which hinders the further increase of density. The kinetic ballooning mode has been suggested to be the responsible mechanism for the fixation of edge density gradients. In the LHW (Lower Hybrid Wave) H-mode density limit discharge, the frequency of type-â…¢ ELMs increased with the growth of density. An ECM (edge coherent mode) occurred in the H-mode and its intensity decreased with the increase of density by SMBI fueling. There was no accumulation of high Z impurities in the whole discharge, and the radiation profile kept rather flat. The coupling of LHW and plasma is very good at high density and the reflection coefficient decrease with the increase of density.The Ohmic and L-mode density limit scaling on EAST is basically in line with Greenwald scaling. However, the H-mode density limit is about 0.8-0.9nGW. It seems that the density limit has a positive correlation with heating power when it is lower. While this relationship became inconspicuous when the heating power increase to a higher level. Density limit is greatly related to the edge cooling of plasma, in which impurities and neutral particles recycling play a very important role. This has been confirmed through statistical analysis of the relation of ne/nGW with Zeff and impurities intensities (Oâ…¡, W and Câ…¢). Besides, most high density discharges were achieved by SMBI fueling, indicating that SMBI is a better fueling method for high density operation. By careful control of impurities and recycling, high density up to 0.93nGW stable H-mode operation was achieved by 1.7 MW LHW and 1.9 MW ICRF with SMBI fueling. It provides a good reference for the long-pulse high density plasma research for EAST and ITER.